In time compression multiplexing (TCM) a single transmission line, say a telephone pair, is utilized for both directions of transmission. The bilaterally transmitted data packets are compressed in time and interleaved. Unlike voice transmission telephony, where on short lines (normally subscriber loops) voice frequency signals are propagating along the line in both directions simultaneously, in TCM a station transmits only after an incoming data packet has cleared the transmission line. Thus transmitter and receiver share the line alternately.
Because the transmission line must be powered to feed the subscriber station, it is necessary to use dc blocking capacitors or transformers to isolate the dc power from the signal transmitter and receiver. It may be advantageous to use capacitors instead of signal transformers due to the smaller size and the lower cost of the former.
Normally the transmission lines will be used in a balanced mode, in order to reduce received common noise, which is rejected by a balanced receiver.
In order to further improve the noise margin, binary signal transmission is preferable to three level signals, for it provides a 6dB advantage.
However, due to the dc blocking capacitance or transformer inductance, certain difficulties arise. The dc blocking component causes an impedance characteristic which produces the well-known exponential decay tail due to the transmitted waveform. The data being received is superimposed on the decay tail and may go undetected. The length of the tail depends also on the density of data "ones" being transmitted.
Due also the random nature of the transmitted data, baseline wander is also a problem. For the data, passing through two sets of dc blocking capacitors (one set at the transmit side and one set at the receive side), sags until the zero line reaches a level coincident with the data average. As the data stream randomly changes, the base or zero line wanders randomly, also causing detection difficulties. The larger are the dc blocking capacitors, the lesser is the baseline wander, but the longer is the decay tail.